When analog signals are measured in systems such as those found in motor vehicles, they are usually measured in a ratiometric manner. FIG. 1 shows a typical system for processing analog signals in motor vehicles. In the figure, line 12 represents the vehicle power supply line, which typically carries a positive DC voltage of approximately 13 volts when the vehicle ignition is on, and zero volts when the vehicle ignition is off. A load resistor 14 is connected in series with sender 28, which has a resistance that varies in relation to a vehicle parameter. An example for sender 28 is the type of variable resistive sender typically used to measure fuel level, oil pressure, engine temperature, etc.. Resistor 14 and sender 28 act as a voltage divider, with the voltage signal on line 18, which may be referred to as the sender output signal, varying in relation to the impedance of sender 28, and therefore in relation to the parameter being measured.
A second voltage divider comprising resistors 20 and 30 is connected between line 18 and ground 34. The second voltage divider couples the signal on line 18 to input line 22 of comparator 24. The signal on input line 22 is proportional to the sender output signal on line 18. The ratio of resistors 20 and 30 ensure that the voltage on line 22 is less than or equal to the supply voltage for comparator 24. Resistor 20 is large enough to protect the input of comparator 22 from voltage transients which would otherwise cause current surges at the input of comparator 24.
A third voltage divider comprising resistors 16 and 32 is connected between line 12 and ground line 34. This third voltage divider sets the reference voltage at the second input line 23 for comparator 24. Comparator 24 may be of the type having an analog output or of the type having a digital output, depending upon the system requirements.
In a systems such as those found in automobiles, the supply voltage on line 12 is subject to a large amount of voltage transients. Regulated power supplies for systems such as those found on vehicles are avoided because of the expense of regulating the whole power supply system and the fact that much of the vehicle electrical systems do not need a regulated supply. In the circuit shown, the fact that the ignition voltage on line 12 may vary does not affect the output line 26 of comparator 24 because the resistor dividers provide signal on lines 22 and 23 that have a constant ratio to one another, as long as the resistors have the same temperature coefficient.
Assuming comparator 24 is in an integrated circuit, according to the prior art, resistors 14, 16, 20, 30 and 32 and sender 28 must all be external components. Resistors 16 and 20 must be external because they are necessary to protect the integrated circuit from current surges. According to the prior art, resistors 30 and 32 must also be external because it is impractical to try to make resistances on integrated circuits that have the same temperature coefficient as the external resistors. If resistors 30 and 32 do not have the same temperature coefficient as resistors 16 and 20, the ratio of the input lines 22 and 23 may no longer be dependent on just resistor 14 and the impedance of sender 28, and will vary with temperature.
What is desired is means for minimizing external components when interfacing ratiometric inputs with integrated circuitry because reducing the number of external components reduces the system cost.